Thinkstock Images/Getty Images(NEW YORK) -- They are supposed to help America's obesity problem: calorie counts boldly displayed on restaurant menus across the country -- important information, considering Americans now eat one-third of their meals outside the home.

Two states and nine counties require them Tuesday, and by the middle of next year, a federal law is expected to force chain restaurants, convenience stores and vending machines nationwide to post calorie counts.

But how accurate are those numbers that so affect your waistline?

A 2011 study by Tufts University sampling food from 42 restaurants says it depends.

Fast food restaurants were the most accurate because of the uniform recipes and portions, but there were wide variations found in sit-down restaurants.

"We found that 20 percent of the foods we tested had 100 calories or more over what was stated on the menu," Lorien Urban, a postdoctoral associate in the energy metabolism lab at Tufts University and first author of the study, told ABC News. "We would consider that to be a considerable amount."

Urban explained that consuming an extra 100 calories per day can lead to an extra 10 pounds in one year.

Most concerning was that a majority of the errors Urban and her colleagues found were made on the diet side of the menu.

"These were the foods that people who are trying to manage their weight would gravitate towards and they may be getting more calories than they expect," she said.

ABC News sent producers in three cities that already require posting menu calories to major chains to do a sampling under the direction of a nationally known lab and found that more than half of the low-cal meals tested had more calories than listed on the menu.

In total 24 food samples from four sit-down restaurants and one McDonald's were collected and the results were surprising.

But the sit-down restaurants had results sometimes wildly different than advertised.

In all, only one calorie count was accurate -- a Skinnylicious chicken salad sandwich from the Cheesecake Factory.

Eleven meals had more calories than on the menu and 10 had fewer calories. Some were over by only 40 calories; another was over by as much as 420 calories, again at the Cheesecake Factory: this time an order of the fish and chips dinner.

Urban said that fast food restaurants tended to be more accurate than sit-down because of the formulaic preparation that fast food restaurants use.

"Things are arriving already packaged into the restaurants and it's just a matter of warming it up and serving it to the consumer," she said. "A sit-down restaurant, things are being prepared on [the] spot [and] by chance some extra butter gets into the pan."

That can change the calorie amount.

All the restaurants and their trade association say that most calorie counts are as accurate as possible and tested extensively to make sure.

They conceded that there are variations, mostly due to portion size and individual restaurant preparation, and that the menus warn actual calories may vary.

What can you do? Take control of what is put on top of the entree by asking for everything fattening -- such as cheeses, sauces or dressings -- on the side.